Dialkyldithiophosphate Acids (HDDPs) as Effective Lubricants of Sol-Gel Titania Coatings in Technical Dry Friction Conditions

The goal of this study was the investigation of the effectiveness of dialkyldithiophosphate acids (HDDPs) films in improving the tribological properties of thin, sol-gel derived titania coatings. Amorphous, anatase, and rutile titania coatings were obtained using sol-gel dip-coating deposition after treatment at 100, 500, and 1,000 A degrees C, respectively. Titania coatings were then modified from the liquid phase by HDDPs acids having dodecyl-(C(12)), tetradecyl-(C(14)), and hexadecyl-(C(16)) alkyl chains deposited by dip-coating (DC) and Langmuir-Blodgett (LB) methods. The influence of the deposition procedure, the length of the HDDPs alkyl chain and the type of titania substrate on the surface morphology and tribological properties were studied. It was found, using wetting contact angle measurements, that these modifications of titania coatings decrease the surface free energy and increase its hydrophobicity. The surface topography imaged by Atomic force microscopy (AFM), exhibit island-like or agglomerate features for the DC deposition method, while smooth topographies were observed for LB depositions. Tribological tests were conducted by means of a microtribometer operating in the normal load range 30-100 mN. An enhancement of tribological properties was observed upon modification, as compared to unmodified titania.